Should I Use Vent Filter for Supply Vent? (The Answer is No)

No. That’s the quick answer. Supply vents do not need – in fact, should not have – an air filter. Why? The filter belongs on the return side of your HVAC’s furnace and AC system. It’s a “must” to have one there. And if you have a return air filter of the right size that is properly installed, then there is no need for a supply vent filter. In fact, it will cause more problems than you might already have. 

Why do people ask this question? 

Because some homeowners have stuff like dust and moldy smells caused by mold spores coming out of their supply vents and want to prevent it from getting into the air they breathe. If that’s the case in your home, our ductwork needs cleaning and sealing. That is the answer to the problem, not masking the problem or making it worse with a vent filter on all supply vents. The way to prevent allergy-causing pollutants from blowing around your home is to have the right air filter on the return side and to have your duct runs properly sealed to prevent them from sucking in dust and other nasty stuff from your attic or basement and blowing it out the supply vents.

Details follow, but here’s the warning: Having a filter on the supply side or both sides will restrict airflow and might do four unwanted things. It might cause mechanical failure, make your home uncomfortably warm in summer or cool in winter, increase the amount of bad stuff in the air and make your energy costs go up, up, up.

Now for the details:

Should I Use Vent Filters on Supply Vents?

The first time we got this question, we wondered, “why would you do that?” But one of our pros pointed out that a lot of people are concerned about allergens floating around the house and causing allergy outbreaks, asthma attacks and generally poor indoor air quality (IAQ).

It clicked immediately. And we have the answer in three parts: Use the appropriate air filter on the return side, where it belongs. Secondly, have your air ducts inspected and sealed. DIY, if you’re handy. And insulate ductwork in unheated or un-cooled space like an attic or garage. Let’s talk about those three.

1. Use a Return Air Filter

Every HVAC system is designed to have a return air filter either in the return air vent or right inside the air handler or furnace. The purpose is to filter/clean the air before it enters the system and makes it dirty. Pick HVAC has created a list of pages about air filters that will assist you in having the right air filter for your home and making sure it is doing its job. We suggest:

2. Seal your Air Ducts

Ductwork leaks in most homes, and this causes two problems. First, air that has been heated ($$$) or cooled ($$$) leaks into places you don’t need it like the attic, basement or garage – wherever the ducts run in non-living space. The $ signs represent 20-30% waste, the figure given by the US Department of Energy. Finding gaps in ductwork joints, places the ducts have come apart and any other failures, and fixing them, is essential to keeping energy use and cost under control.

Secondly, leaky ducts allow dust, mold spores and other allergens / pollutants to be sucked into the supply vents. This is why you might have dust and other junk blowing out of your registers and vents.

Have a pro seal your ductwork or do it yourself. Try these pages for how-to details:

3. Insulate your Ductwork for Lower Energy Costs

While not related to the question of a vent filter on the supply side, insulating your ducts is a great idea with immediate ROI. There is evidence that ductwork sealing and insulation will pay for itself in lower bills within a few years. And then you’re saving money every single month and using less energy too.

Dirty Ducts – What to Do?

What if there is mold, dust and other pollutants in ductwork? Supply vent filters won’t solve the problem, and they might make it worse.

Having the proper air filter on the return side of your HVAC system and sealing ductwork will keep out additional dust and other unwanted stuff like pollen, mold spores, pet hair and dander, dust mites, rodents and their droppings. But what about the stuff that’s already in there? Putting those cut-and-fit air vent filters won’t help. They just trap the bad stuff in your ducts, and it is better to get rid of it.

While we don’t recommend ductwork cleaning when ductwork is in good condition and a quality air filter has been used, there are times when it is necessary. If your vents are blowing out dust and debris and/or there are musty, moldy smells coming from them, then having the duct runs vacuumed out and sanitized might be necessary. BUT – sorry to shout – but we want to make one point clear: Talk to an HVAC professional rather than a duct cleaning service about the problem.

We say this because an HVAC technician is better-trained and more knowledgeable about ductwork and properly cleaning it. They will have the right solution. How much does duct cleaning cost? This AC Duct Cleaning Cost has the answer.

And when your ducts are clean, make sure to have them sealed to prevent future nasty problems inside them.

Supply Vent Air Filters – What Can Go Wrong?

We’ve shown that your supply vent isn’t designed for a vent filter. The system should have a filter on the return side. If there is one there, as there should be, adding a vent filter to each vent will cause several problems.

1. Your pollution and allergy problem will get worse.

Is there dust, mold and other debris in your ductwork? Have the ducts cleaned. A filter in the supply duct will hold the unwanted stuff in your ducts where mold and bacteria might get started or multiply. These allergens and pollutants will make their way out of the ducts somewhere and get into your airspace. The problem will worsen.

2. Airflow will be restricted, and your HVAC equipment will suffer.

Your furnace or AC air handler has a blower motor designed to push air through a certain amount of restriction caused by an air filter. If the restriction is increased, the motor will work harder. Your AC or heat pump compressor will work harder. These costly parts will wear more quickly or might suddenly fail.

3. Your system might get too hot or too cold, and additional mechanical problems could occur.

If the system can’t adequately push out heat, the furnace might get too hot. This could lead to a cracked heat exchanger, a mechanical problem best solved, in our opinion, by full furnace replacement. This is discussed in more detail in our Cost of Heat Exchanger Replacement.

While cooling, the indoor coil might become freezing cold, literally. When that happens, humidity building up on it freezes, and the coil becomes covered in ice. This could ruin the coil. At the very least, it will prevent your AC from working properly. And when the ice melts, you could have leaking and water damage.

4. Your energy costs will rise.

A system working too hard uses more energy. And energy costs money. It’s that simple.

Cleaner Air in Your Home

OK, your ducts are cleaned and sealed, and you’re using the right air filter on the return side.

But someone is still sneezing, wheezing and suffering other allergy or asthma symptoms.

As far as indoor air quality is concerned, we recommend two things.

1. Use a filter with the right MERV rating for your home.

MERV rating is a measurement of how effectively the filter removes allergens and other pollutants from the air.

Our guide called Recommended MERV Rating explains how to choose a filter with the filtering ability to remove the allergens and pollutants found in your home.

2. Consider a HEPA air purifier.

For most people, having a filter with a MERV rating between 8 and 10 is sufficient. For those with allergies or asthma, a filter with a MERV rating of 11-13 is advisable. While the proper air filter on your HVAC/AC/furnace will do a lot to remove unwanted material from the air in your home, it can’t do it all.

If your ducts are clean and you’ve got a quality filter of the right MERV rating in your equipment, and you’re still suffering allergy symptoms, an air purifier with a HEPA filter is a good next step.

What is a HEPA filter? It’s one with a MERV rating of 17-20, higher than standard furnace and AC filters are capable of. You wouldn’t want to put a filter that is that restrictive into your HVAC system anyway. It would cause the mechanical issues discussed earlier.

Heating and AC Airflow – Why a Return Air Filter is the Right Design

This section is informational and designed for homeowners that are unfamiliar with the workings of their HVAC system.

Btw, what is HVAC? It stands for Heating, Ventilation and Air Conditioning.

Air Circulation in a Forced Air System

Air filters are used in forced air heating and air conditioning systems. The term “forced air” simply means the unit has a motor-driven blower fan.

The fan is housed in the furnace, if that’s the device you use for heating, or in the air handler if you have a heat pump system supplying your heating and AC.

Supply and Return: There are two sets of ducts connected to your furnace or air handler – supply ducts and one or more return air ducts.

Now, think about a box fan/window fan. It sucks or pulls air in from the back and blows it out the front. Similarly, a blower fan pulls in air through the return air vents and ducts, and it pushes or blows it out through the supply ducts / vents after it has been heated or cooled.

Return Air Filters: The reason filters are designed for the return side is obvious – to keep dust, pollen, mold spores, etc. out of your furnace or air handler where it can do a bunch of damage to the blower, coil and even the outdoor compressor by making it work too hard.

Where does all that stuff come from? Mostly, it is pulled in through the return vents aka cold air return vents. Secondly, it gets sucked into the ductwork through leaks, as discussed above.

We hope this was useful information. If so, consider sharing a link on social media for the benefit of your friends and followers! And feel free to comment or ask a question below. Thanks for reading – and enjoy many of the other insightful pages from the HVAC pros at Pick HVAC!

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